1. Field of the Invention
The present invention relates generally to the interconnection techniques of electronic components or devices (e.g., surface mount components and/or devices) or functional modules with conductive joints. More particularly, the invention relates to a solder ball used for mechanical and electrical interconnection between opposing electrodes of electronic components/devices, functional modules, and substrates, and an interconnection structure with conductive joints formed by the solder balls.
2. Description of the Related Art
When electronic components/devices and/or various types of functional modules (all of which may be referred simply xe2x80x9celectronic components/devicesxe2x80x9d hereinafter) are mounted on a substrate such as a Printed Wiring Board (PWB) with solder balls, the height of conductive joints formed by solder balls needs to be as uniform as possible over the whole surface of the substrate. However, this is difficult for conventional solder balls to be satisfied. This is because conventional solder balls are formed by simply shaping a solder material into spherical bodies and therefore, they are likely to be deformed in their melting and solidifying processes (i.e., their reflowing process).
Moreover, the operation tests for the electronic components/devices, which are carried out before mounting the components/devices on the substrate, are usually conducted after the formation of solder bumps on the bonding pads of the components/devices by melting the solder balls and solidifying them. In these tests, electrical interconnection between the testing apparatus and the components/devices in question is usually achieved by contacting the connecting pins of the testing socket with the solder bumps on the components/devices with a specific pressing force. Additionally, since the operation tests are conducted for each of the components/devices with different testing apparatuses, pressing force is repeatedly applied to the solder bumps on the components/device. Accordingly, with the conventional solder balls, the solder bumps fixed onto the components/devices are likely to be deformed with the pressing force applied repeatedly during the tests and as a result, defective electrical interconnection tends to occur between the pins and the opposing bumps.
To overcome this problem, an improved solder ball has been developed and actually used. The improved solder ball comprises a metal core and a solder layer covering the outer surface of the core. The core is made of metal having a higher melting point than the solder material of the solder layer.
When mounting an electronic component/device on a substrate with the improved solder balls, first, the solder layers covering the metal cores of the solder balls are melted and solidified, thereby forming ball-shaped solder bumps on the corresponding pad or electrode of the component/device. The bumps are fixed to the opposing pads with the solder layers. Next, the component/device is aligned to the substrate in such a way that the bumps thus formed on the component/device contact the corresponding bonding pads or electrodes on the substrate. Thereafter, the component/device and the substrate are heated for a while to selectively melt only the solder layers of the bumps and then, they are cooled to solidify the solder layers thus melted. As a result, the solder layers thus solidified are fixed to the corresponding pads on the component/device and the opposing pads on the substrate while holding the metal cores within the corresponding solder bumps.
In this way, conductive solder joints are formed by the solder layers and their metal cores, thereby mechanically and electrically interconnecting the pads on the component/device with the corresponding pads on the substrate. Thus, the component/device is surface-mounted on the substrate and electrically connected thereto.
As explained above, with the prior-art improved solder balls, the height of the conductive solder joints can be kept substantially uniform over the whole surface of the substrate by the metal cores. Moreover, the cores enhance the mechanical strength of the solder bumps themselves. Therefore, defective electrical interconnection occurring in the operation tests can be avoided.
Generally, if the interconnection structure formed by using solder balls is applied to electronic equipment (e.g., computer systems) where the mounting or packaging density of electronic components/devices is becoming higher and higher, the dimension (i.e., the diameter) of solder balls must be decreased as the interval and/or width of the bonding pads become smaller.
With the prior-art improved solder ball having a metal core therein, to realize or obtain a desired quantity of solder material (i.e., a desired volume of the solder layer) with a small solder ball, the ratio of the thickness of the solder layer to the diameter of the metal core needs to be larger. In this case, however, the thickness of the solder layer tends to be non-uniform, in other words, the volume of the solder layer tends to fluctuate. Thus, there is a problem that a desired quantity of solder material is not obtainable stably.
If the thickness of the solder layer is decreased according to the diameter of the metal core, the thickness of the solder layer will be approximately uniform, thereby suppressing the fluctuation of the volume of the solder material. In this case, however, the volume of the solder layer decreases. This means that a desired quantity of solder material is not obtainable. As a result, a sufficient interconnection strength of the solder joint is not obtained as desired, because the adhesion strength of the solder layer to the bonding pad or electrode is insufficient. This leads to degradation or deterioration of mechanical and electrical interconnection reliability of the solder joint.
Accordingly, an object of the present invention is to provide a solder ball that achieves a desired quantity of solder material for a solder joint easily without increasing the thickness of a solder layer formed to cover a core.
Another object of the present invention is to provide a solder ball that achieves a desired quantity of solder material for a solder joint stably even if a solder ball is smaller.
Still another object of the present invention is to provide a solder ball that enhances the adhesion strength of a solder layer to a core thereof.
A further object of the present invention is to provide an interconnection structure using solder balls that ensures the mechanical and electrical interconnection reliability of solder joints as desired even if electronic components/devices are mounted on a substrate or an electronic component/device at a higher mounting or packaging density.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the invention, a solder ball is provided, which comprises:
a conductive core having depressions on its outer surface; and
a solder layer formed to cover the outer surface of the core in such a way as to fill the depressions of the core.
With the solder ball according to the first aspect of the invention, the conductive core has the depressions on its outer surface and the solder layer is formed to cover the outer surface of the core in such a way as to fill the depressions of the core. Therefore, the volume of the solder layer is larger than that of the above-described prior-art improved solder ball without depressions on the core by the total volume of the depressions. In other words, the quantity of the solder material included in the ball is supplemented by the solder material filled into the depressions. Accordingly, even if the thickness of the solder layer is not increased, a desired quantity of solder material for a solder joint is obtainable easily.
Moreover, since the thickness of the solder layer can be decreased according to the total volume of the depressions, the solder layer will be formed uniformly. This means that the volume of the solder layer is restrained from fluctuating. Thus, a desired quantity of solder material for a solder joint is obtainable stably even if the solder ball is made smaller.
Furthermore, part of the solder layer is filled into the depressions of the core and therefore, the adhesion strength of the solder layer to the core is enhanced compared with the above-described prior-art improved solder ball without depressions on the core.
In a preferred embodiment of the solder ball according to the first aspect of the invention, the core has a higher melting point than the solder layer. The core has wettability to the solder layer. In this embodiment, there is an additional advantage that the outer surface of the core is easily covered with and adhered to the solder layer.
In another preferred embodiment of the solder ball according to the first aspect of the invention, the core has a cavity in its inside. In this embodiment, there is an additional advantage that the weight of the solder ball is reduced, in other words, a lightweight solder ball can be realized. It is preferred that the depressions do not reach the cavity. Preferably, the core is shell-shaped by the cavity.
In a still another preferred embodiment of the solder ball according to the first aspect of the invention, the core and the cavity are spherical and concentric with each other. The core is shell-shaped.
In a further preferred embodiment of the solder ball according to the first aspect of the invention, the depressions are directed from the outer surface of the core to approximately a center thereof.
In a still further preferred embodiment of the solder ball according to the first aspect of the invention, each of the depressions has a depth larger than a diameter of a mouth thereof.
In a still further preferred embodiment of the solder ball according to the first aspect of the invention, the core has a cavity in its inside, and each of the depressions has a depth smaller than a diameter of a mouth thereof.
In still further preferred embodiment of the solder ball according to the first aspect of the invention, the core is made of a porous metal body having pores. Part of the pores reaches the outer surface of the core, forming the depressions. In this embodiment, there is an additional advantage that the solder ball is fabricated easily, because the process of forming the depressions of the core is eliminated.
According to a second aspect of the invention, an interconnection structure using solder balls is provided, which is designed for mechanically and electrically interconnecting a first electrode formed on a first member with an opposing second electrode formed on a second member with a conductive joint. The joint is formed by melting and solidifying the solder ball according to the first aspect of the invention. Specifically, the solder ball comprises a conductive core having depressions on its outer surface, and a solder layer formed to cover the outer surface of the core in such a way as to fill the depressions of the core.
With the interconnection structure according to the second first aspect of the invention, the joint is formed by melting and solidifying the solder ball according to the first aspect of the invention. Therefore, because of the same reason as described for the solder ball according to the first aspect of the invention, even if the dimension of the solder ball is reduced, a desired quantity of solder material for the joint is obtainable easily.
Thus, even if the dimension of the solder ball is reduced, the adhesion strength of the solder layer to the first and second electrodes will not be insufficient. This means that sufficient adhesion strength is obtainable as desired. As a result, the mechanical and electrical interconnection reliability of the joint is ensured as desired even if electronic components/devices are mounted on a substrate at a higher mounting or packaging density.
In a preferred embodiment of the structure according to the second aspect of the invention, the core of the solder ball has a higher melting point than the solder layer. The core has wettability to the solder layer.
In another preferred embodiment of the structure according to the second aspect of the invention, the core has a cavity in its inside. In this embodiment, it is preferred that the depressions do not reach the cavity. Preferably, the core is shell-shaped.
In a still another preferred embodiment of the structure according to the second aspect of the invention, the core and the cavity are spherical and concentric with each other. The core is shell-shaped.
In a further preferred embodiment of the structure according to the second aspect of the invention, the depressions are directed from the outer surface of the core to approximately a center thereof.
In a still further preferred embodiment of the structure according to the second aspect of the invention, each of the depressions has a depth larger than a diameter of a mouth thereof.
In a still further preferred embodiment of the structure according to the second aspect of the invention, the core has a cavity in its inside, and each of the depressions has a depth smaller than a diameter of a mouth thereof.
In still another preferred embodiment of the structure according to the second aspect of the invention, the core is made of a porous metal body having pores. Part of the pores reaches the outer surface of the core, forming the depressions.